JP3323690B2 - Optical wavelength division multiplexing communication equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength division multiplexing communication device, and more particularly to an optical wavelength division multiplexing communication device capable of suppressing the influence of interference due to four-wave mixing to increase the transmission capacity of an optical fiber communication system. Things.

[0002]

2. Description of the Related Art An optical fiber communication system using an optical wavelength division multiplexing signal can increase its transmission capacity without changing a transmission line, and is therefore applied to a future basic optical fiber communication system. Is expected technology.
In the case of using an optical wavelength division multiplexing signal, if four-wave mixing exists between signal wavelengths, if the signal wavelength intervals are equal, four-wave mixing generated from two other signal lights adjacent to the signal light interferes. And limits the characteristics of the system, such as the maximum transmittable distance and the maximum number of multiplexable signals. In order to avoid the characteristic deterioration due to the four-wave mixing, a technique of making the wavelength interval between any two wavelengths unequal is known (F.
Forghieri et. Al., IEEE Photon. Technol. Lett. Vo
l.6, no.6, pp.754-756).

One example will be described with reference to FIG.
FIG. 10 shows the wavelength of signal light allocated to each of the eight channels on the horizontal axis. As is clear from the figure, the wavelength interval between the two wavelengths is 0.8 nm for the first and second channels, 0.9 nm for the second and third channels, and 0.9 nm for the third and third channels. The wavelength interval from the fourth channel is 1.
.., And the wavelength intervals between any two wavelengths are unequal.

[0004] By making the intervals unequal, it is possible to effectively suppress interference due to four-wave mixing generated from another signal light adjacent to a certain signal light.

[0005]

However, if the wavelength spacing between any two wavelengths is made unequal, the required optical signal band becomes wider than in the case of equal spacing, so that the number of multiplexable wavelengths is increased. However, there is a problem that the number is reduced.
The reason is that the minimum wavelength interval of a wavelength division multiplexed signal is naturally limited by the performance of an optical demultiplexer used for signal separation of a wavelength division multiplexed signal. In this case, the band of nx is sufficient, whereas in the case of the unequally spaced system, each wavelength interval must be larger than x, so that the band at the time of n-wavelength multiplexing becomes considerably larger than nx.

In the example of FIG. 10, the minimum wavelength interval x is x =
Since the wavelength is 0.7 nm, if the wavelength interval between the two wavelengths is equal, the optical signal band of the first to eighth channels is 4.9 nm. On the other hand, if the wavelength interval between any two wavelengths is made unequal, an optical signal band of 7 nm is required, and the optical signal band becomes about 1.4 times.

In addition to the technique of making the signal wavelength intervals unequal, the generation efficiency of four-wave mixing is high when the polarization states of the optical signals involved in generation are the same, and low when they are orthogonal. There is known a technique in which the polarization states of adjacent optical signals are intentionally orthogonally arranged by utilizing the above.

However, the state of polarization between adjacent signals is
When the optical fiber is transmitted over a certain distance due to the birefringence, the optical fiber is not maintained. Therefore, when applied to a long-distance wavelength multiplexing system, it is insufficient as a four-wave mixing generation suppression technique. Further, when the polarization states of adjacent optical signals are made orthogonal, the polarization states coincide between signal wavelengths that are one wavelength apart, so that there is a problem that the generation efficiency of four-wave mixing is rather increased.

SUMMARY OF THE INVENTION It is an object of the present invention to eliminate the above-mentioned problems of the prior art and to reduce the required optical signal band while suppressing the signal-to-signal interference caused by four-wave mixing in a wavelength division multiplexed optical communication system. An object of the present invention is to provide an optical wavelength division multiplexing communication device capable of increasing the number of possible wavelengths.

[0010]

To achieve the above object, the present invention provides an optical wavelength division multiplexing communication apparatus for optically multiplexing and transmitting signals of a plurality of different wavelengths.
Five or more optical transmitters that output a signal of a wavelength whose wavelength interval between wavelengths is used again as a wavelength interval between two other wavelengths that separate another signal by two or more wavelengths and that has no periodicity in the wavelength interval. comprising an optical wavelength multiplexing transmitting terminal station including, the optical wavelength multiplexing transmission
The terminal station determines whether the polarization state of adjacent signal wavelengths is
The polarization state is controlled to be orthogonal at the output end of the transmitting terminal.
It is characterized in that it comprises a polarization state control means for controlling .

[0011]

[0012]

[Effect] The generation efficiency of four-wave mixing is proportional to the light intensity of each signal involved in the generation of four-wave mixing and inversely proportional to the wavelength interval between each signal wavelength (Reference: N. Shibata et. Al., El.
ectronics Letters, vol. 24, pp. 1528-1529, 1988).
Roughly speaking, when the wavelength interval is doubled, the generation efficiency of four-wave mixing is reduced to 1/4. Therefore, increasing the wavelength interval is effective in suppressing the generation of four-wave mixing. In an optical wavelength division multiplexing communication system in which optical signals of five or more wavelengths are multiplexed, a wavelength interval between certain two wavelengths is reused as a wavelength interval between another two wavelengths separating another optical signal by two or more wavelengths. In consideration of the above, four-wave mixing caused by the same wavelength interval in two places is suppressed by a large wavelength interval caused by separating another optical signal by two or more waves,
Deterioration on the optical signal transmission characteristics can be effectively reduced. Further, since the wavelength multiplexing signal band can be reduced by reusing the wavelength multiplexing interval, the maximum multiplexable wavelength number can be increased.

When the polarization states of the optical signals adjacent to each other at the output end of the optical wavelength division multiplexing transmission terminal are made orthogonal, the transmission optical fiber adjacent to the transmission terminal has a distance of several hundred km. Since the occurrence of four-wave mixing between signals is suppressed, further improvement can be achieved.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 2 is a block diagram showing the configuration of one embodiment of the present invention. In the figure, T is an optical wavelength multiplexing transmitting terminal, and R is an optical wavelength multiplexing receiving terminal. The optical wavelength multiplexing transmitting terminal T
Comprises optical transmitters 1 to 8 of the first to eighth channels and a multiplexer 9. On the other hand, the optical wavelength multiplexing receiving terminal R includes optical receivers 11 to 18 of the first to eighth channels and a duplexer 19. Although the present embodiment is an example in which eight optical transmitters and eight optical receivers are provided, the present invention is not limited to this, and it suffices that each has five or more optical transmitters and optical receivers. The optical wavelength multiplexing transmitting terminal T and the optical wavelength multiplexing receiving terminal R are connected by a transmission optical fiber 21, and the transmission optical fiber 2
1 is provided with optical amplifiers 22 at predetermined intervals.

FIG. 3 is a block diagram showing a specific example of the optical transmitter 1. As shown in FIG. The optical transmitter 1 includes a light source 1a, an external modulator 1
b and an output terminal 1c. Light source unit 1a
Is composed of a light emitting section, a drive current control section, a temperature control section, and the like. The current and temperature used to drive the light emitting unit are
The driving current control unit and the temperature control unit stabilize with high accuracy. For example, the driving current is 10 μA, and the temperature is 0.1 μA.
Stabilized to about 01 ° C. The optical transmitters 2 to 8 have the same configuration.

In this embodiment, the wavelengths of the signal lights output from the optical transmitters 1 to 8 are such that the wavelength interval between certain two wavelengths is the wavelength interval between other two wavelengths separating another signal by two or more. It has been determined that it will be used again and that the wavelength spacing will not be periodic. For example, it is determined as shown in FIG. That is, the wavelengths of the signal lights output from the optical transmitters 1 to 8 (first to eighth channels) are 155, respectively.
4.9, 1555.7, 1556.6, 1557.7,
1558.5, 1559.5, 1560.7, and 1
It has been determined to be 561.6 nm.

As apparent from FIG. 1, the wavelength spacing between the first and second channels and between the fourth and fifth channels two channels away from the second channel is 0.8 nm, The wavelength intervals between the three channels and between the seventh and eighth channels four channels away from the third channel are set to 0.9 nm.

The signal lights of the respective wavelengths output from the optical transmitters 1 to 8 (see FIG. 2) are multiplexed by a multiplexer 9, passed through a transmission optical fiber 21, and received by an optical wavelength multiplex receiver. It is transmitted to the terminal station R. The signal light is transmitted while repeating the attenuation in the transmission optical fiber 21 and the amplification by the optical amplifier 22. Upon receiving the signal light, the optical wavelength division multiplexing receiving terminal R splits the signal light by the demultiplexer 19 and sends each signal light to the optical receivers 11 to 18.

In the present embodiment, the wavelengths of the signal lights output from the optical transmitters 1 to 8 are set to the values described with reference to FIG. 1 and the same wavelength interval occurs at two places. Since the four-wave mixing is suppressed by a large wavelength interval generated by separating two or more other optical signals, deterioration on optical signal transmission characteristics can be effectively reduced. As is apparent from a comparison between FIG. 10 and FIG. 1, the optical signal band of the first to eighth channels of this embodiment is 6.7 nm.
(= 1561.6-1554.9), which is 7 in FIG.
nm, and the amplification band of the optical amplifier 22 can be used effectively. In other words, the maximum multiplexable wavelength number can be increased.

Next, a second embodiment of the present invention will be described. FIG. 4 shows the configuration of the optical wavelength multiplexing transmitting terminal T of the present embodiment. Other configurations are the same as or equivalent to those in FIG. In the figure, 31 is a multiplexer for odd channels, 32 is a multiplexer for even channels, and 33 is a multi-polarizer. The odd-numbered channel combiner 31
Optical transmitters 1 of the first, third, fifth and seventh channels,
The signal lights output from 3, 5, and 7 are multiplexed. Further, the multiplexers 32 of the even-numbered channels include the second, fourth, and sixth multiplexers.
And multiplex the signal lights output from the optical transmitters 2, 4, 6 and 8 of the eighth channel. FIG. 5 shows the polarization state arrangement of the signal light of each channel obtained by the polarization multiplexer 33. That is, the polarization state of the odd-numbered channel signal light and the polarization state of the even-numbered signal light are orthogonal to each other.

Next, the operation of this embodiment will be described. As in the first embodiment, the wavelength of the signal light output from each of the optical transmitters 1 to 8 of the first to eighth channels is such that the wavelength interval between certain two wavelengths is two or more waves apart from other signals. It is used again as a wavelength interval between another two wavelengths, and is determined so that the wavelength interval has no periodicity. The signal lights output from the odd-numbered channel optical transmitters 1, 3, 5 and 7 are multiplexed by the multiplexer 31, and the even-numbered channel optical transmitters 2, 4, and 6 are combined.
The signal lights output from the multiplexors 8 are multiplexed by the multiplexer 32 and sent to the polarization multiplexer 33. Polarized multi-wave device 3
3 generates a state in which the polarization state of the odd-numbered channel and the polarization state of the even-numbered channel are orthogonal to each other, and outputs the state to the transmission optical fiber 21. As a result, in the transmitting terminal station, the adjacent channels have orthogonal polarization states.

In general, four-wave mixing does not occur between signal lights having different polarization states. Therefore, according to the present embodiment, four-wave mixing that occurs between adjacent channels can be suppressed. When the signal wavelength of each channel is determined as shown in FIG. 1, four-wave mixing that occurs between the even-numbered channels and the odd-numbered channels whose polarization states match does not cause interference in other channels. In addition, the effect of four-wave mixing generated every other channel can be suppressed.

On the other hand, if the wavelength multiplexing intervals are made equal as in the conventional apparatus (see FIG. 6), four-wave mixing that occurs between the even-numbered channels and the odd-numbered channels whose polarization states match each other is not possible. The characteristic is degraded because it causes interference to other channels. The distance at which the orthogonality of the signal polarization state is maintained is determined by the birefringence of the transmission optical fiber and the wavelength multiplexing interval.

Experiments were actually performed with the signal wavelength arrangement as shown in FIG. 1 and the equidistant arrangement as shown in FIG. 6 to confirm the effect of this embodiment. The transmission optical fiber 21 is 66 per section.
km, 15 optical amplifiers were used, and a 1000 km experimental transmission line was constructed. The optical bandwidth for setting the signal wavelength is
In this embodiment, since the band is limited by the band reduction due to the multi-repeat of the optical amplifier, the maximum is about 9 nm.
And the signal wavelength arrangement as shown in FIG.

When a signal arrangement as shown in FIG. 6 is used,
Even if a sufficient power such as -15 dBm is applied to the optical receiver, only a code error rate of about ^10.sup.-6 can be achieved in the channel having the best characteristics after the transmission of 1000 km. On the other hand, when the signal arrangement of FIG. 1 was used, the bit error rate characteristics as shown in FIG. 7 were obtained. As can be seen from the figure, even after 1000 km transmission, a good bit error rate of 10 ^-9 or less is achieved with a relatively weak input of an optical receiver input power of -30 to -25 dBm in all channels. It was confirmed that the effect of this example was remarkable.

Next, a third embodiment of the present invention will be described. FIG. 8 shows the configuration of the optical wavelength multiplexing transmitting terminal T of the present embodiment. Other configurations are the same as or equivalent to those in FIG. In the figure, 41 to 48 are first to fourth.
The polarization state controllers of the eighth channels 1 to 8 are 49
An 8-channel multiplexer, 21 is a transmission optical fiber, and the other reference numerals are the same as or equivalent to those in FIG. The polarization state controllers 41 to 48 include first to eighth channels 1 to
No matter which two signal wavelengths are selected, setting is made such that their polarization states do not match.

FIG. 9 shows an example of the polarization state arrangement of each channel in this embodiment. When the signal polarization state arrangement as shown in FIG. 9 is performed, unlike the second embodiment, four-wave mixing occurs between any signal wavelengths. However, according to this embodiment, four-wave mixing does not occur as much as four-wave mixing that occurs between odd channels and even channels in the case of the second embodiment. Therefore, the characteristics are averaged in all the channels, and the characteristics are improved in the whole system.

[0028]

As is apparent from the above description, according to the present invention, a plurality of optical transmitters are arranged such that the wavelength interval between certain two wavelengths is between two other wavelengths separating another signal by two or more waves. Since a signal of a wavelength that is used again as a wavelength interval and has no periodicity in the wavelength interval is output, it is possible to effectively reduce deterioration of optical signal transmission characteristics and increase the maximum number of multiplexable wavelengths. Can be.

Further, when the polarization states of the adjacent optical signals are made orthogonal at the output end of the optical wavelength division multiplexing transmission terminal, the transmission of the optical fiber adjacent to the transmission terminal at a distance of several hundred km can be achieved. Since the generation of four-wave mixing between signals is suppressed, the transmission characteristics of the optical communication system can be further improved.

Even if a plurality of optical transmitters extract any two signal wavelengths, there is no case where the polarization states of the two signals coincide at the output end of the optical wavelength division multiplexing transmission terminal. Since the polarization state is controlled, the transmission characteristics are averaged in all the channels, so that the transmission characteristics of the optical communication system can be improved when viewed as a whole system.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an example of an optical wavelength multiplex signal arrangement according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a system configuration of a first embodiment of the present invention.

FIG. 3 is a block diagram illustrating a configuration of a specific example of an optical transmitter according to the present embodiment.

FIG. 4 is a block diagram showing a configuration of an optical wavelength division multiplexing transmitting terminal according to a second embodiment of the present invention.

FIG. 5 is an explanatory diagram illustrating a polarization state of an optical signal obtained by a second embodiment.

FIG. 6 is an explanatory diagram of an example of a conventional optical wavelength multiplex signal arrangement.

FIG. 7 is a diagram illustrating the measurement of the bit error rate characteristics of each channel for explaining the experimental result based on the second embodiment.

FIG. 8 is a block diagram showing a configuration of an optical wavelength division multiplexing transmitting terminal according to a third embodiment of the present invention.

FIG. 9 is an explanatory diagram illustrating a polarization state of an optical signal obtained by a third embodiment.

FIG. 10 is an explanatory diagram of an example of a conventional optical wavelength multiplex signal arrangement.

[Explanation of symbols]

T: optical wavelength multiplexing transmitting terminal, R: optical wavelength multiplexing receiving terminal, 1
8 to 1 to 8 channel optical transmitters 9 to 1
1 to 18: first to eighth channel optical receivers, 19: duplexer, 21: transmission optical fiber, 22: optical amplifier, 31,
Reference numeral 32 denotes a multiplexer, 33 denotes a polarization-multiplexed multiplexer, 41 to 48: a polarization state controller, and 49 a multiplexer.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shu Yamamoto 2-3-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo International Telegraph and Telephone Corporation (72) Inventor Shigeyuki Akiba 2-3-Nishi Shinjuku, Shinjuku-ku, Tokyo No. 2 International Telegraph and Telephone Corporation (56) References JP-A-8-171109 (JP, A) JP-A-7-264166 (JP, A) Kazuhiro Oda, et al. , 10-channel x 10-G bit / s optical FDM transmission over a 500-km dispersion-shifted fiber employingune, OFC'95, 1995, p. 27-29, TuH1 (58) Fields investigated (Int. Cl. ⁷ , DB name) H04J 14/00-14/08 H04B 10/00-10/28 JICST file (JOIS)

Claims (4)

(57) [Claims] 1. An optical wavelength division multiplexing communication apparatus for transmitting signals of a plurality of wavelengths different from each other by optical wavelength division multiplexing, wherein a wavelength interval between certain two wavelengths is different from another wavelength between two other wavelengths. It used again as the wavelength intervals, and includes an optical wavelength multiplexing transmitting terminal station comprising five or more optical transmitters for outputting a wavelength signal of no periodicity in the wavelength interval, wherein the optical wavelength multiplexing transmitting terminal is adjacent Polarization of the signal wavelength
Are orthogonal at the output end of the WDM transmitting end station
An optical wavelength division multiplexing apparatus comprising a polarization state control means for controlling the polarization state as described above . 2. The optical wavelength multiplex communication apparatus according to claim 1 , wherein said polarization state control means multiplexes an output signal of an optical transmitter of an odd channel of said optical wavelength multiplex transmission terminal. , A second multiplexer for multiplexing the output signals of the even-numbered channel optical transmitters, and polarization states of the signals from the first and second multiplexers such that the polarization states are orthogonal to each other. An optical wavelength division multiplexing communication device comprising: a polarization multi-wavelength wave controller for controlling a wave state. 3. The optical wavelength division multiplexing communication device according to claim 1, wherein the optical wavelength division multiplexing transmission terminal sets the polarization state of the two signals to be equal to the optical wavelength even if two signal wavelengths are removed. An optical wavelength division multiplexing communication apparatus comprising: a polarization state control means for controlling a polarization state so that no coincidence occurs at an output terminal of a multiplex transmission terminal. 4. The optical wavelength division multiplexing communication device according to claim 3 , wherein the polarization state control means comprises: a plurality of polarization state controllers for polarizing output signals of the optical transmitter of each channel by a predetermined angle; An optical wavelength multiplexing communication device, comprising: a multiplexer for multiplexing signals output from the respective polarization state controllers.